1. Field of the Invention
The present invention relates to an oxygen concentration detector which is used for air/fuel ratio control, etc. in an automobile engine.
2. Description of the Related Art
Oxygen concentration detectors have conventionally been provided with protecting covers to protect their sensing elements. That is, an oxygen concentration detector has a solid electrolyte, a sensing element which consists of the solid electrolyte coated on the surface with an outer electrode, a heater provided inside the above-mentioned solid electrolyte, and a protecting cover which protects the above-mentioned sensing element. In addition, two levels of openings are provided in the above-mentioned protecting cover, through which a gas to be measured is introduced into the gas-measuring chamber.
The oxygen concentration detector described above is sometimes mounted onto the exhaust pipe, etc. and used as a part of the engine""s combustion control system, in order to make the automobile engine burn fuel at the theoretical air/fuel ratio. The exhaust gas cleaning efficiency is highest when the engine burns fuel at the theoretical air/fuel ratio.
However, in order to further increase the exhaust gas cleaning efficiency, it has been attempted, in recent years, to raise the sensitivity of the oxygen concentration detectors used for detecting the air/fuel ratio.
The sensing elements of such oxygen concentration detectors, however, have unstable characteristics until heated above the activation temperature. As a result, when the temperature of the sensing element is low, for example, when the engine is started, the oxygen concentration sensitivity is also low.
Consequently, in order to increase the exhaust gas cleaning efficiency, it is necessary to heat the sensing element to the above-mentioned activation temperature within a short time after starting the engine.
To meet this requirement, it has been proposed to heat the sensing element by increasing the power fed to the heater to increase the heater temperature.
However, in such cases there is a risk that the temperature of the heat-generating part inside the heater may increase to an abnormally high temperature which is considerably higher than the temperature necessary for oxygen concentration detection, and may even rise above the heat-resistant temperature of the ceramic or other material of which the heater is constructed. This results in problems such as damage to the heater and the shortening of its useable life.
In light of these problems, it is an object of the present invention to provide an oxygen concentration sensing element which allows the sensing element to be heated to the activation temperature more rapidly, without raising the temperature of the heat-generating part above the heat-resistant temperature of the heater.
The present invention is an oxygen concentration detector which comprises a sensing element which includes a solid electrolyte coated on the surface thereof with an outer electrode, a heater provided inside or near the above-mentioned solid electrolyte, and a protecting cover which protects the above-mentioned sensing element, characterized in that
the protecting cover has two levels of openings, and
the area of the outer electrode which contributes to the exchange (conduction) of oxygen ions is constructed within the range defined by the length of the heat-generating part of the heater, and the section of the protecting cover adjacent (nearest) to the sensing element is provided with two levels of openings in the axial direction outside the range corresponding to the range in which the outer electrode contributes to exchange of oxygen ions, wherein the relationship between the length L1 of the heat-generating part and the distance between the openings L2 of the two levels of openings in the axial direction is such that L1/L2=0.9-1.3.
The above-mentioned two levels of openings may be coaxial with respect to the central axis of the oxygen concentration detector, or they may be non-coaxial, at spirally offset positions. The distance between the two levels of openings is the distance from the perimeter line including the lower edge of the openings at the proximal end of the sensing element to the perimeter line including the upper edge of the openings at the distal end of the sensing element (see FIGS. 7(A), (B)).
The heater consists of a heat-generating part which increases in temperature upon electrification to heat the sensing element, a lead part which supplies power to the heat-generating part, and a ceramic body which houses the heat-generating part and the lead part. The heat-generating part and the lead part may take any of a variety of shapes (see FIG. 8, FIGS. 15-18), and the construction of the present invention may be applied to heat-generating parts and lead parts of all such shapes.
The heat-generating part is made of a paste consisting of, for example, platinum, tungsten, molybdenum or the like. Furthermore, the heater may either be provided inside the sensing element as a separate heater, or it may be formed integrally with the solid electrolyte of the sensing element (which results in a layered sensing element).
The relationship between the length L1 of the heat-generating part and the distance between the openings L2 is such that L1/L2=0.9-1.3. If the value of L1/L2 is less than 0.9, then there is a possibility that the temperature of the heat-generating part may rise above the heat-resistant temperature of the heater when the sensing element is heated to the activation temperature. This may result in damage to the heater and the shortening of its useable life.
On the other hand, if the value exceeds 1.3 the power consumption of the heat-generating part may increase, thus lowering efficiency.
A lower power consumption is also preferred since the heater has a positive resistance temperature coefficient, and thus in cases where the heater resistance is high, sufficient power sometimes may not be provided by the voltage of the battery mounted in the automobile.
The length of the heat-generating part is preferably between 8 mm and 16 mm. If the length is less than 8 mm, then the temperature of the heat-generating part may rise above the heat-resistant temperature of the heater when the sensing element is heated to the activation temperature. Conversely, a length exceeding 16 mm will increase power consumption by the heat-generating part and may result in the same type of problem as when the value of L1/L2 is greater than 1.3.
The distance between the openings L2 is preferably between 9 mm and 16 mm. If the length is less than 9 mm, then the shorter distance between the outer electrode of the sensing element and the openings may result in a greater tendency toward deterioration of the outer electrode due to contamination in the gas to be measured. Conversely, if the length exceeds 16 mm then the longer distance between the outer electrode of the sensing element and the openings may result in a poor response.
The axial offset xcex94L between the center position of the heat-generating part of the heater and the center position of the distance between the openings L2. is preferably no more than L2/4.
If this xcex94L is longer than L2/4, as explained below, the sensing element will be cooled by the low-temperature gas to be measured which is introduced through the openings, and this may make it impossible to rapidly heat the sensing element to the activation temperature.
The area of the individual openings in the openings is preferably between 0.75 and 3.5 mm2, and the total area counting both levels of openings is preferably between 10 and 23 mm2. If the total area of both levels of openings is less than 10 mm2, then it may become impossible to introduce a sufficient amount of the gas to be measured into the gas-measuring chamber to allow detection of the oxygen concentration, and if the area of each individual opening is less than 0.75 mm2, then working of the openings becomes problematic, resulting in disadvantages from the viewpoints of workability and productivity.
Conversely, if the total area of the openings of both levels is greater than 23 mm2, then cooling of the sensing element when the low-temperature gas to be measured is introduced may render it impossible to reach the activation temperature without excessively raising the temperature of the heat-generating part. If the area of each opening is greater than 3.5 mm2, condensed water in the exhaust gas pipe may pass through the opening so that the element is damaged or cracked by contact of water.
The above-mentioned outer electrode is provided as a band around the surface of the solid electrolyte, and the outer electrode preferably does not face the openings of the above-mentioned protecting cover.
That is, most limiting current-type oxygen concentration detectors have the outer electrode on only specific sections of the sensing element. Also, since the outer electrode is the most essential part for oxygen concentration detection, this section must be kept at the activation temperature for stable detection of the oxygen concentration.
Consequently, situating the above-mentioned openings and outer electrode in the manner described above makes it possible to prevent lowering of the temperature of the outer electrode when the low-temperature gas to be measured is introduced. An additional effect is that since it is possible to avoid direct contact between the outer electrode and the gas to be measured which is introduced through the openings, there may be realized a reduction in electrode malfunction and loading of the diffusion layer due to toxins in the gas.
The surface of the outer electrode may also have a gas diffusion-resistant layer. Providing a limiting current-type oxygen concentration detector with such a gas diffusion-resistant layer will allow adjustment of the number of oxygen molecules and oxygen ions taken into the sensing element, to obtain the desired output.
The exterior of the above-mentioned protecting cover may be provided with an external cover which has throughholes. Water droplets infiltrating through the openings in the protecting cover cause water cracks in the sensing element, and contamination in the gas to be measured results in its deterioration. Thus, providing an external cover on the exterior of the protecting cover will effectively protect the sensing element from such damage. An opening may be provided at the bottom of the cover for the sensor element, which may improve the responsibility of the sensor.
In the oxygen concentration detector of the present invention, the section of the outer electrode of the sensing element which contributes to the exchange of oxygen ions is constructed within the area defined by the length of the heat-generating part of the heater.
As a result, the heat from the heater is received over the entire surface of the outer electrode. Thus, the heater is able to preferentially heat the sections most useful for detection of oxygen concentration, and thus heat the sensing element to the activation temperature without causing the temperature of the heat-generating part to rise above the heat-resistant temperature of the heater.
In addition, the specific relationship described above is set between the length L1 of the heat-generating part and the distance between the openings L2 of the two levels of openings in the axial direction.
This allows the sensing element to receive the heat from the heat-generating part in the most efficient manner. As a result, since the heater can heat the sensing element more efficiently, the sensing element may be heated to the activation temperature by a heater with a lower temperature.
The present invention is most effective when the temperature of the gas to be measured is considerably lower than the activation temperature of the sensing element.
Thus, according to the present invention there is provided an oxygen concentration detector which allows the sensing element to be heated to the activation temperature more rapidly, without raising the temperature of the heat-generating part above the heat-resistant temperature of the heater.